1. Field of the Invention
The present invention relates to methods of preparing a compound semiconductor crystal and to the compound semiconductor crystals prepared thereby, and particularly to methods of preparing a carbon-containing, compound semiconductor crystal and compound semiconductor crystals obtained thereby.
2. Description of the Background Art
It has been conventionally well known that as for an LEC method using a stainless chamber there is a correlation between the CO gas concentration provided in the chamber and the carbon concentration of a GaAs crystal in a high-pressure Ar gas ambient.
FIG. 3 is a graph of carbon concentration in a GaAs crystal versus CO gas concentration in a LEC furnace found in Advanced Electronics Series I-4 Bulk Crystal Growth Technology, Keigo Hoshikawa, BAIFUKAN, p.184, Fig. 7.22. FIG. 3 shows that carbon concentration in a GaAs crystal and CO gas content in the LEC furnace are correlated by a straight line. In the LEC method, the correlation represented in the graph is applied to the adjustment of carbon concentration in a GaAs crystal. The carbon concentration in a GaAs crystal can be controlled by adjusting the CO gas content in the ambient gas using a CO gas cylinder and an Ar gas cylinder for dilution connected to the stainless chamber.
FIG. 4 shows an exemplary GaAs crystal growth equipment for the LEC method disclosed in Japanese Patent Laying-Open No. 1-239089. Referring to FIG. 4, Japanese Patent Laying-Open No. 1-239089 discloses a method of preparing a single crystal of compound semiconductor by placing in a predetermined gas ambient a raw-material housing portion housing a raw-material melt, detecting at least the concentration of one of H2, O2, CO2 and CO corresponding to components of the ambient gas, and controlling the detected concentration of a component at a predetermined value to keep over the entirety of an ingot a predetermined concentration of a residual impurity mixed into a resulting single crystal.
This method can, however, not be applied in preparing a compound semiconductor crystal in a gas-impermeable air-tight vessel incapable of supplying a gas from outside the airtight vessel, such as a quartz ampoule.
Japanese Patent Laying-Open No. 3-122097 discloses a method of preparing a GaAs crystal in a quartz ampoule wherein a carbon source is arranged internal to the ampoule and external to a crucible in fluid communication with a polycrystalline compound provided as a raw material to allow the GaAs crystal to be doped with carbon. “Fluid communication” means a free flow of vapor and heat between the inside and outside of the crucible which allows carbon to be transferred into the crucible and thus to a melt. In accordance with the method, a carbon disk is arranged on an opening of a cap. It discloses that the ingots of various doped levels can be provided by varying the amount of carbon arranged external to the opening and/or the crucible.
With this method, however, a large amount of carbon source is placed above the melt. Thus fine powder of carbon falls thereon and can thus vary the carbon concentration thereof. Particularly, the controllability can be poor at a slight carbon concentration corresponding to a level of 0.1×1015 cm−3 to 2×1015 cm−3.
Japanese Patent Laying-Open No. 64-79087 discloses a method of preparing a single crystal of GaAs doped with carbon to reduce dislocation, using a reactor or a boat for crystal growth at least partially formed of carbon. It discloses that when a graphite boat is used, a part of the carbon boat changes into a gas (CO or CO2) due to oxygen derived from a small amount of As2O3, Ga2O or the like remaining in the quartz reactor and the gas is thus added to the single crystal of GaAs in synthesis reaction or in single-crystal growth.
In accordance with this method, however, it is difficult to control the carbon concentration in the crystal due to the difficulty of controlling the amount of As2O3, Ga2O or the like remaining in the quartz reactor. In particular, the controllability can be poor at a slight carbon concentration corresponding to a level of 0.1×1015 cm−3 to 2×1015 cm−3.
Japanese Patent Laying-Open No. 2-48496 discloses a method of preparing a Cr-doped, semi-insulating GaAs crystal wherein a quartz boat or a quartz crucible is used to grow the crystal under the existence of nitrogen oxide or carbon oxide. It discloses that when a GaAs crystal is grown under the existence of nitrogen oxide or carbon oxide, the oxide serves as an oxygen doping source to reduce the Si concentration of the grown crystal so that a semi-insulating crystal is reliably provided.
However, this method contemplates control of oxygen concentration and does not describe control of carbon concentration.